Fibers crosslinked in substantially individualized form and various methods for making such fibers have been described in the art. The term "individualized, crosslinked fibers", refers to cellulosic fibers that have primarily intrafiber chemical crosslink bonds. That is, the crosslink bonds are primarily between cellulose molecules of a single fiber, rather than between cellulose molecules of separate fibers. Individualized, crosslinked fibers are generally regarded as being useful in absorbent product applications. In general, three categories of processes have been reported for making individualized, crosslinked fibers. These processes, described below, are herein referred to as (1) dry crosslinking processes, (2) aqueous solution crosslinking processes, and (3) substantially non-aqueous solution crosslinking processes. The fibers themselves and absorbent structures containing individualized, crosslinked fibers generally exhibit an improvement in at least one significant absorbency property relative to conventional, uncrosslinked fibers. Often, this improvement in absorbency is reported in terms of absorbent capacity. Additionally, absorbent structures made from individualized crosslinked fibers generally exhibit increased wet resilience and increased dry resilience relative to absorbent structures made from uncrosslinked fibers. The term "resilience" shall hereinafter refer to the ability of pads made from cellulosic fibers to return toward an expanded original state upon release of a compressional force. Dry resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a substantially dry condition. Wet resilience specifically refers to the ability of an absorbent structure to expand upon release of compressional force applied while the fibers are in a moistened condition. For the purposes of this invention and consistency of disclosure, wet resilience shall be observed and reported for an absorbent structure moistened to saturation.
Processes for making individualized, crosslinked fibers with dry crosslinking technology are described in U.S. Pat. No. 3,224,926 issued to L. J. Bernardin on Dec. 21, 1965. Individualized, crosslinked fibers are produced by impregnating swollen fibers in an aqueous solution with crosslinking agent, dewatering and defiberizing the fibers by mechanical action, and drying the fibers at elevated temperature to effect crosslinking while the fibers are in a substantially individual state. The fibers are inherently crosslinked in an unswollen, collapsed state as a result of being dehydrated prior to crosslinking. Processes as exemplified in U.S. Pat. No. 3,224,926, wherein crosslinking is caused to occur while the fibers are in an unswollen, collapsed state, are referred to as processes for making "dry crosslinked" fibers. Dry crosslinked fibers are characterized by low fluid retention values (FRV). It is suggested in U.S. Pat. No. 3,440,135, issued to R. Chung on Apr. 22, 1969, to soak the fibers in an aqueous solution of a crosslinking agent to reduce interfiber bonding capacity prior to carrying out a dry crosslinking operation similar to that described in U.S. Pat. No. 3,224,926. This time consuming pretreatment, preferably between about 16 and 48 hours, is alleged to improve product quality by reducing nit content resulting from incomplete defibration.
Processes for producing aqueous solution crosslinked fibers are disclosed, for example, in U.S. Pat. No. 3,241,553, issued to F. H. Steiger on Mar. 22, 1966. Individualized, crosslinked fibers are produced by crosslinking the fibers in an aqueous solution containing a crosslinking agent and a catalyst. Fibers produced in this manner are hereinafter referred to as "aqueous solution crosslinked" fibers. Due to the swelling effect of water on cellulosic fibers, aqueous solution crosslinked fibers are crosslinked while in an uncollapsed, swollen state. Relative to dry crosslinked fibers, aqueous solution crosslinked fibers as disclosed in U.S. Pat. No. 3,241,553 have greater flexibility and less stiffness, and are characterized by higher fluid retention value (FRV). Absorbent structures made from aqueous solution crosslinked fibers exhibit lower wet and dry resilience than pads made from dry crosslinked fibers.
In U.S. Pat. No. 4,035,147, issued to S. Sangenis, G. Guiroy and J. Quere on July 12, 1977, a method is disclosed for producing individualized, crosslinked fibers by contacting dehydrated, nonswollen fibers with crosslinking agent and catalyst in a substantially nonaqueous solution which contains an insufficient amount of water to cause the fibers to swell. Crosslinking occurs while the fibers are in this substantially nonaqueous solution. This type of process shall hereinafter be referred to as a nonaqueous solution crosslinked process; and the fibers thereby produced, shall be referred to as nonaqueous solution crosslinked fibers. The nonaqueous solution crosslinked fibers disclosed in U.S. Pat. No. 4,035,147 do not swell even upon extended contact with solutions known to those skilled in the art as swelling reagents. Like dry crosslinked fibers, they are highly stiffened by crosslink bonds, and absorbent structures made therefrom exhibit relatively high wet and dry resilience.
Crosslinked fibers as described above are believed to be useful for lower density absorbent product applications such as diapers and also higher density absorbent product applications such as catamenials. However, such fibers have not provided sufficient absorbency benefits, in view of their detriments and costs, over conventional fibers to result in significant commercial success. Commercial appeal of crosslinked fibers has also suffered due to safety concerns. The most widely referred to crosslinking agent in the literature, formaldehyde, unfortunately causes irritation to human skin and has been associated with other human safety concerns. Removal of free formaldehyde to sufficiently low levels in the crosslinked product such that irritation to skin and other human safety concerns are avoided has been hindered by both technical and economic barriers.
Crosslinked fibers made from crosslinking agents other than formaldehyde may also have residual levels of crosslinking agent which are higher than preferred for applications in the vicinity of human skin. Therefore, it may be desirable to treat the fibers subsequent to crosslinking in such a way to reduce the level of unstable, residual crosslinking agent on the fibers. One method for doing this is to wash the fibers with water subsequent to crosslinking. This method is effective, but does not reduce the level of residual crosslinking agent as low as may be desirable. Additionally, such post-crosslinking washes necessitate additional capital and operating costs associated with the washing and drying of the fibers.
It is an object of this invention to provide commercially viable individualized crosslinked fibers and absorbent structures made from such fibers, as described above, which can be safely utilized in the vicinity of human skin.
It is additionally an object of this invention to provide a process for making individualized, crosslinked fibers having low levels of residual crosslinking agent wherein capital investment for equipment and related operating costs are minimized.